Imagine if aspiring surgeons could coast through medical school by simply showing up to class and watching the clock tick away. Or if mechanics could receive their certifications by just observing, not repairing, cars.

Students in a third grade technology class play a math-related computer game at St. John Paul II Catholic Academy in Boston, Massachusetts on Feb. 25, 2016. (Scott Eisen/Bloomberg)

Crazy? Of course. But this is reality in most American schools, where student progress is pegged to age and the number of hours spent in a classroom.

It's no wonder why students are falling behind. While 90% of parents believe their children are reading at or above grade level, national assessments show that less than 40% of students actually are. It's time to stop measuring student success by the number of minutes they log in class, and instead start measuring actual learning achievements. Without such a shift, schools will continue to deny students the skills and knowledge required for success.

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Under the traditional K-12 teaching model, every student is expected to move through lessons at the same pace, regardless of how well they've mastered a given topic. As a result, even a "B" student might miss as much as 20% of a class' content—content that may be integral to understanding future lessons.

Advanced students don't fare any better under the current system. Consider a traditional eighth-grade English class; it always starts in September and always ends in June. So if a student masters her course material by the end of March, she is left twiddling her thumbs until the next school year.Such stories are common. Three in four teachers think high-achieving students are "bored and under-challenged" in school, according to a survey from the Thomas B. Fordham Institute.

The truth is that students don't all learn at the same speed. It's time for schools to adapt their educational techniques to account for this. That's the central insight behind mastery-based education.